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Introduction

This documentation covers the Subject Interface v1, an open protocol for communication between Cooperative Intelligent Transportation System (C-ITS) participants.

TLC Terminology

The interface originally served Traffic Light Controllers (TLCs), and this legacy terminology persists in the API specifications for backward compatibility. "TLC" now refers to any connected entity: traffic controllers, sensors, barriers, displays, or any C-ITS participant. See Terminology Reference for the mapping between legacy and generic terminology.

What is the Subject Interface?

The Subject Interface defines a vendor-neutral, license-free communication protocol for interoperability across C-ITS ecosystems. It provides data exchange between roadside devices, central systems, and service providers through a dual-layer architecture:

  • JSON-REST API Layer - Administrative control and lifecycle management
  • TCP Streaming Protocol Layer - Low latency data exchange

Key Features

  • Low latency communication for C-ITS applications
  • Bidirectional data exchange with continuous asynchronous streaming
  • Message-agnostic architecture supporting any payload type
  • Role-based access control with security framework
  • Horizontal scaling support for distributed deployments

Introduction Contents

Communication Patterns

Roadside-to-central and central-to-central communication patterns, supported entity types, and system topology.

Documentation Structure

This documentation is organized into main chapters:

Architecture

System architecture, components, security framework, and role-based access control.

REST API

JSON-REST API reference including session management, registration, and token operations.

TCP Streaming Protocol

Technical specifications for the streaming protocol including datagram types and connection management.

Example

Complete implementation walkthrough demonstrating both API and streaming protocol usage.

Getting Started

For system integrators and developers new to the Subject Interface:

  1. Review this introduction to understand the interface and communication patterns
  2. Study Architecture for system architecture, security, and access model
  3. Implement using REST API and TCP Streaming Protocol specifications
  4. See Example for a complete implementation walkthrough